The present invention relates generally to the arts of data communications and telephony, and more particularly, to a filter system (and associated methodology) for connection between a source of large-amplitude, high-frequency noise and at least one broadband communications channel which is frequency-division multiplexed into a communications medium of a communications system.
With the increasing bandwidth demands from the advent of the Internet, service providers have looked for ways to increase data performance over the two-wire, copper twisted-pair, transmission lines that connect the telephone central offices (COs) to the customer premises (CPs). The telephone system architecture connects customer premises equipment (CPE) to CO switches over transmission lines known as xe2x80x9clocal loops,xe2x80x9d xe2x80x9csubscriber loops,xe2x80x9d xe2x80x9cloops,xe2x80x9d or the xe2x80x9clast milexe2x80x9d of the telephone network. These loops are part of the cable plant that provides the physical means by which transmission services are implemented. To allow connectivity between COs, the CO switches are interconnected to each other over high-capacity, multiplexed transmission facilities known as trunk lines. Unlike the CO-to-CO trunk connections, which have generally been digital for some time, digital service provision to the customer premises is a more recent development. These digital CO-to-CO trunk connections are part of a telephone network architecture known as integrated services digital network (ISDN).
Historically, the PSTN (public switched telephone network) evolved with analog, subscriber loops connected to a telephone network with circuit-switched capabilities which were designed to carry voice communications. The evolution of the telephone network from a system just designed to carry analog, voice communications to a system which could carry digital voice and data led to the development of the ISDN architecture. One of the goals of the ISDN architecture is to provide customers (or subscribers) with integrated access (over a single loop) to the services of a digital network.
Because of the costs of replacing or supplementing the cable plant to the customer premises, technologies are needed that utilize existing subscriber loops to provide digital connectivity to the customer. Furthermore, to provide customers with additional flexibility and enhanced services, multiplexing techniques are needed to fill a physical subscriber loop with multiple logical channels. In contrast to the ubiquitous use of frequency-division multiplexing (FDM) and/or time-division multiplexing (TDM) on CO-to-CO trunk connections, the standard analog POTS (plain old telephone service) capability provided over subscriber loops generally has not been multiplexed with any other channels. To solve the problems of digital connectivity and multiple channels of communication, subscriber loops capable of carrying digital channels have been developed and are known as digital subscriber lines (DSLs). Logical channels within a subscriber line which carry digital signals are known as DSL channels while logical channels within a subscriber line which carry POTS signals are known as POTS channels.
Frequency-division multiplexing modulates (or frequency shifts) signals to different frequency channels before placing them on the communications medium. This modulation of signals to higher frequency ranges creates a broadband signal which has been frequency shifted to a different frequency band than the base band frequencies (or baseband) of the signal. A communications channel associated with a broadband signal is known as a broadband channel, while a communications channel associated with a baseband signal is known as a baseband channel. The equipment that MOdulates and DEModulates signals onto a communications medium is called a MODEM. Broadband is an adjective often used to describe equipment, networks, and systems that use FDM to modulate signals, while narrowband is an adjective often used to describe equipment, networks, and systems that do not modulate signals out of their base frequency bandwidth range. Digital signals that utilize square waveforms theoretically use an infinite frequency spectrum which limits the practicality of using FDM to multiplex other signals onto the same communications medium with a square waveform signal. In contrast to FDM, the bandwidth of a communications medium is broken up into time-slots in TDM.
One of the initial DSL solutions is the service of narrowband ISDN which has several drawbacks. For example, although the narrowband ISDN BRI (Basic Rate Interface) service communicates three logical channels over a subscriber loop, it does not allow direct compatibility with POTS because the narrowband ISDN channels occupy the baseband frequencies (0-4 KHz) that are used by POTS to carry a voice frequency (VF) channel. Because narrowband ISDN uses the frequency baseband of POTS, voice service over a narrowband ISDN DSL requires the use of actively powered terminal adapters (TAs) to provide POTS functionality by utilizing one of the logical channels in the narrowband ISDN DSL.
As a result of some of the drawbacks from narrowband ISDN, and as a result of the customer demand for increased bandwidth, newer DSL (digital subscriber line) technologies have been developed. Some of these newer DSL technologies use frequency-division multiplexing and/or time-division multiplexing to provide a higher-bandwidth digital service over a local subscriber loop without significantly interfering with standard POTS functionality. This backward compatibility with standard POTS functionality offers easier and lower cost customer migration to the newer DSL technologies without the previously mentioned drawbacks of narrowband ISDN. The newer DSL technologies accomplish this functionality by modulating their broadband, digital signal above (at higher frequencies than) the 0 KHz to 4 KHz baseband of standard, analog POTS signals. This FDM capability to provide multiple channels on a single subscriber line will work with both baseband and broadband analog signals as well as with baseband and broadband digital signals. However, the common existence of older, legacy, analog POTS loops and equipment coupled with the demand for newer, digital service has caused the development of new DSL technologies that allow a new, digital channel to be frequency-division multiplexed above an old, analog POTS channel. Several variants of new DSL technology exist, e.g., ADSL, SDSL, RADSL, VADSL, etc., with such variants generally referred to collectively as xDSL. Generally, when multiple channels are communicated across an xDSL connection, a POTS splitter is utilized to decouple the frequency-division multiplexed channels. A POTS splitter usually is situated at the CO as well as at the customer premises.
Although the transmission of both digital broadband and analog POTS baseband signals over a subscriber loop between a CO telephone switch and a customer premises offers many potential advantages for customers, several practical problems must be solved in implementing DSL solutions. One significant problem results from the high-frequency transient noise signals generated by the legacy, analog POTS equipment connected to the subscriber loop. These transient signals from the POTS equipment are in frequency ranges above the POTS baseband and have components which interfere with the one or more broadband channels. Many functions of the legacy POTS equipment generate these high-frequency noise components.
Historically, the POTS subscriber loop was designed with the functions needed to communicate both baseband, voice-conversation signals and call-state/line-state signaling information (known generally as subscriber loop signaling or POTS loop signaling as opposed to CO-to-CO trunk signaling). Although POTS is a service deployed by the telephone company, it also can be viewed as defining an interface standard that specifies how to communicate both baseband, analog voice conversations and subscriber loop signaling information. Because call-state/line-state signaling messages are different for a CO switch and a telephone, a CO switch usually implements a CO-side or office-side POTS interface while a telephone usually implements a subscriber-side POTS interface. POTS equipment that generates call-state/line-state signaling messages is usually configured to implement either an office-side POTS interface or a subscriber-side POTS interface. In general, the POTS interface defines a multi-point bus transmission line that is connected to one device implementing an office-side POTS interface and a plurality of devices, each implementing a subscriber-side POTS interface.
Call-state/line-state signaling messages are used in a subscriber loop for various frequently-used signaling functions. Generally, the CO switch uses subscriber loop signaling to notify the customer premises about events in the telephone network, while customer premises equipment (CPE) uses subscriber loop signaling to inform the CO to perform actions for the customer. These POTS subscriber loop signaling functions often generate high-frequency interference with broadband channels. For instance, the on-hook/off-hook signal and the pulse-dialing signal are square waveforms which have high-frequency components and theoretically require infinite bandwidth. Also, the off-hook/on-hook signal and the touch-tone dial signal are usually implemented on telephones by spring-loaded, electromechanical switches. Furthermore, the ringing signal is often switched onto the subscriber loop by electromechanical relays. Like most electromechanical devices, these relays do not have perfect response functions, and may not be synchronized to the ringing signal zero crossing. Thus, as opposed to the electromechanical devices reacting in a clean, sharp manner to on and off activations, these devices physically bounce after being activated with an on-off transition and before reaching a final, steady-state condition. As a result, the signal generated on the subscriber loop by these devices has high-frequency components created by the bouncing of the electromechanical relays and the spring-loaded switches. The non zero switching of the ringing signal on to the subscriber loop generates wide band noise. Relay bounce exacerbates the switching and resultant noise problems.
The POTS subscriber loop signaling functions are a source of undesirable noise resulting from transients and harmonics at frequency ranges above the 0 KHz to 4 KHz bandwidth of the standard POTS baseband. The high-frequency components of this noise may interfere with the one or more frequency-division multiplexed broadband channels in frequency ranges above the POTS baseband. Because the legacy, analog POTS subscriber loops are not frequency-division multiplexed with additional channels, the transient noises did not affect communications on these subscriber loops.
Unlike many of the other signals communicated on a subscriber loop, the ring signal and its associated noise transients generated by the telephone switch have a high amplitude that creates a significant problem to broadband channels utilizing the subscriber loop. The ring signal initiated by the telephone switch is a large-amplitude, periodic waveform that is communicated for two second intervals, with each ring separated by four second time periods. This POTS ring signal is specified in FCC Part 68 and generally is an alternating current (AC) signal having a root-mean-squared (rms) voltage of up to about 150 volts. The ring signal was designed to have a large magnitude in order to properly actuate electromechanical ringers that were present in earlier versions of telephones.
Although the large-amplitude ring signal and its associated noise transients are a historical legacy of POTS subscriber loops, on broadband DSLs, these transient ringing signal noises with frequencies above the POTS baseband result in a lower signal-to-noise ratio (SNR) on the broadband channels. By suppressing the transient ringing signal noises, a broadband communications channel will have an improved signal-to-noise ratio which can be used to modify the design parameters of a communications system. Those skilled in the art will be aware that many various design tradeoffs could be made to take advantage of an improved signal-to-noise ratio.
Thus, a heretofore unaddressed need exists in the industry for a way to prevent or substantially minimize the adverse affects of a large-amplitude, high-frequency signal, such as the ring signal, upon the one or more broadband channels that are communicated along the subscriber loop with the analog voice channel.
The present invention relates generally to the arts of data communications and telephony, and more particularly, to a filter system (and associated methodology) for connection between a source of large-amplitude, high-frequency noise and at least one broadband communications channel which is frequency-division multiplexed into a communications medium. The large-amplitude, high-frequency noise commonly results from a telephone ring signal originated by telephone ring generation equipment (e.g., a CO telephone switch). Generally, the communications medium is a telephone connection transmission line (e.g., a subscriber loop) that can communicate information signals over a baseband channel which usually provides POTS (plain old telephone service) to a subscriber. The filter system is used for suppressing transients and harmonics on the connection caused by large-amplitude, high-frequency signals, such as the telephone ring signal. The suppression advantageously minimizes adverse effects on the one or more broadband channels on the connection.
The present invention comprises a filter system having an amplitude-dependent transfer function that suppresses the interference from at least one signal component, the interference being imposed on at least one communications channel, the at least one signal component being at least part of a signal generated by a noise source, and the at least one signal component being outside of a specified amplitude range.
In another sense, the present invention may be viewed as providing a method for suppressing transients and harmonics on a telephone connection that communicates a baseband, analog POTS channel and one or more broadband channels. These transients and harmonics are generated along with a telephone ring signal and should be minimized to reduce the adverse effects on the one or more broadband channels on the connection.
In this regard, the method can be broadly viewed as comprising the following steps: implementing an amplitude-dependent transfer function; and suppressing the at least one signal component that is outside of a specified amplitude range by using the amplitude-dependent transfer function.
Other features and advantages of the present invention will become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional features and advantages be included herein within the scope of the present invention.